This really depends on how long you are working out for and what your goals are.
If you are training for more that 60-90 minutes and are trying to gain muscle mass, then consider ingesting a quick digesting carbohydrate or protein. This should be something low in fibre as the aim is to get a quick boost of energy and fast absorption into the body. Liquid sources are convenient and easy to digest so a sports drink of combined protein and carb shakes are a good option.
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